1. Field of the Invention
The present invention relates to a capacitor used for assisting power sources of various types of electronic, electric, and mobile devices.
2. Background Art
In recent years, a large number of electric double layer capacitors with a small size and large capacity have been used as capacitors for backing up a microprocessor, memory, and timer; and for assisting various types of power sources. An electric double layer capacitor accumulates energy in an electric double layer formed at an interface between the polarizable electrode and electrolyte.
FIGS. 7A and 7B are respectively a sectional view and an exploded perspective view of a conventional electric double layer capacitor. The capacitor includes case 101, capacitor element 102, positive electrode terminal 103, and negative electrode terminal 104. Case 101 is formed of, for example, insulative resin or ceramic. Capacitor element 102 is formed of positive electrode 102A, negative electrode 102B, and separator 102C which are laminated. Capacitor element 102 together with an electrolyte (not shown) is contained inside case 101. A first end of positive electrode terminal 103 is connected to positive electrode 102A, and a first end of negative electrode terminal 104 is connected to negative electrode 102B. A second end of terminal 103 and a second end of terminal 104 are drawn outside penetrating through a wall of case 101. In FIG. 7B, terminal 103 and terminal 104 are in a state before being bent along the outer surface of case 101, namely in a plain plate.
Thus, with this electric double layer capacitor, positive electrode terminal 103 and negative electrode terminal 104 are drawn outside penetrating through case 101. In this structure, an electrolyte inside case 101 is likely to leak outside through gaps between each terminal and through holes of case 101. For this reason, a part of the wall of case 101 through which terminals 103 and 104 penetrate is roughened. In this way, rough-surfaced part 105 formed at a position where terminals 103 and 104 contact case 101 increases adherence between case 101 and each terminal and enlarges the contact area. This structure reduces the possibility that an electrolyte leaks outside.
An electric double layer capacitor, however, generates a large amount of gas inside case 101 while being used. In this way, an increase in the inner pressure of case 101 can cause exfoliation at a part (rough-surfaced part 105) where terminals 103 and 104 contact case 101. Consequently, an electrolyte is still likely to leak outside through gaps between each terminal and case 101.
In detail, the following mechanism works. That is, an increase in the inner pressure of case 101 causes case 101 to expand vertically to be deformed. Consequently, the top and bottom surfaces of case 101 undergo an outward-expanding stress, resulting in the side of case 101 warped outside more largely at its rim than its substantially central part. The interface between case 101 and terminals 103, 104 undergoes a stress in a direction in which case 101 exfoliates from terminals 103, 104, which produces gaps at the interface between case 101 and terminals 103, 104 (both are made of different materials). The electrolyte contained in case 101 is likely to enter these gaps and to gradually leak from the inside to the outside of case 101.
Particularly with recent electric double layer capacitors, flat and relatively thin capacitor elements are used and the wall thickness of case 101 is thinned for downsizing and lower profile. In such a case, case 101 is deformed more prominently, and thus an electrolyte leaks easily.